BackgroundDepression is the most prevalent cause of illness-induced disability worldwide. Face-to-face psychotherapeutic interventions for depression can be challenging, so there is a need for other alternatives that allow these interventions to be offered. One feasible alternative is Internet-based psychological interventions. This is the first randomized controlled trial (RCT) on the effectiveness of an Internet-based intervention on depression in primary health care in Spain.ObjectiveOur aim was to compare the effectiveness of a low-intensity therapist-guided (LITG) Internet-based program and a completely self-guided (CSG) Internet-based program with improved treatment as usual (iTAU) care for depression.MethodsMulticenter, three-arm, parallel, RCT design, carried out between November 2012 and January 2014, with a follow-up of 15 months. In total, 296 adults from primary care settings in four Spanish regions, with mild or moderate major depression, were randomized to LITG (n=96), CSG (n=98), or iTAU (n=102). Research completers at follow-up were 63.5%. The intervention was Smiling is Fun, an Internet program based on cognitive behavioral therapy. All patients received iTAU by their general practitioners. Moreover, LITG received Smiling is Fun and the possibility of psychotherapeutic support on request by email, whereas CSG received only Smiling is Fun. The main outcome was the Beck Depression Inventory-II at 3 months from baseline. Mixed-effects multilevel analysis for repeated measures were undertaken.ResultsThere was no benefit for either CSG [(B coefficient=-1.15; P=.444)] or LITG [(B=-0.71; P=.634)] compared to iTAU, at 3 months. There were differences at 6 months [iTAU vs CSG (B=-4.22; P=.007); iTAU vs LITG (B=-4.34; P=.005)] and 15 months [iTAU vs CSG (B=-5.10; P=.001); iTAU vs LITG (B=-4.62; P=.002)]. There were no differences between CSG and LITG at any time. Adjusted and intention-to-treat models confirmed these findings.ConclusionsAn Internet-based intervention for depression combined with iTAU conferred a benefit over iTAU alone in the Spanish primary health care system.Trial RegistrationClinicaltrials.gov NCT01611818; https://register.clinicaltrials.gov/prs/app/action/SelectProtocol? selectaction=Edit&uid=U0001NPQ&ts=2&cx=gctdh2&sid=S0003KJ6 (Archived by WebCite at http://www.webcitation.org/6jbsUvUDz)